The field of the present invention relates to electric-field-driven spinning of polymer nanofibers (e.g., electrospinning) or spraying of small droplets (e.g., electrospraying). In particular, fluid formulations for electric-field-driven (EFD) spinning with relatively reduced fluid conductivity are disclosed herein.
The subject matter disclosed herein may be related to subject matter disclosed in co-owned U.S. non-provisional application Ser. No. 11/634,012 entitled “Electrospraying/electrospinning array utilizing a replacement array of individual tip flow restriction” filed Dec. 5, 2006 in the names of John A. Robertson and Ashley Steve Scott (now U.S. Pat. No. 7,629,030), said application being incorporated by reference as if fully set forth herein.
“Electrospinning” and “electrospraying” refer to the production of, respectively, so-called “nanofibers” or “nanodroplets”, which may be “spun” as fibers or “sprayed” as droplets by applying high electrostatic fields to one or more fluid-filled spraying or spinning tips (i.e., nozzles or spinnerets). The high electrostatic field typically (at least when using a relatively conductive fluid) produces a Taylor cone at each tip opening from which fibers or droplets are ejected. The sprayed droplets or spun fibers are typically collected on a target substrate. A high voltage supply provides an electrostatic potential difference (and hence the electrostatic field) between the spinning tip (usually at high voltage) and the target substrate (usually grounded). A number of reviews of electrospinning have been published, including (i) Huang et al, “A review on polymer nanofibers by electrospinning and their applications in nanocomposites,” Composites Science and Technology, Vol. 63, pp. 2223-2253 (2003), (ii) Li et al, “Electrospinning of nanofibers: reinventing the wheel?”, Advanced Materials, Vol. 16, pp. 1151-1170 (2004), (iii) Subbiath et al, “Electrospinning of nanofibers,” Journal of Applied Polymer Science, Vol. 96, pp. 557-569 (2005), and (iv) Bailey, Electrostatic Spraying of Liquids (John Wiley & Sons, New York, 1988). Details of conventional electrospinning materials and methods can be found in the preceding references and various other works cited therein, and need not be repeated here.
Conventional fluids for electrospinning (melts, solutions, colloids, suspensions, or mixtures, including many listed in the preceding references) typically possess significant fluid conductivity (e.g., ionic conductivity in a polar solvent, or a conducting polymer). Fluids conventionally deemed suitable for electrospinning have conductivity typically between 100 μS/cm and about 1 S/cm (Filatov et al; Electrospinning of Micro-and Nanofibers; Begell House, Inc; New York; 2007; p 6). It has been observed that electrospinning of nanometer-scale fibers using conventional fluids typically requires conductivity of about 1 mS/cm or more; lower conductivity typically yields micron-scale fibers. In addition, conventional methods of electrospinning typically include a syringe pump or other driver/controller of the flow of fluid to the spinning tip, and a conduction path between the high voltage supply and the fluid to be spun. Such arrangements are shown, for example, in U.S. Pat. Pub. No. 2005/0224998 (hereafter, the '998 publication), which is incorporated by reference as if fully set forth herein. In FIG. 1 of the '998 publication is shown an electrospinning arrangement in which high voltage is applied directly to a spinning tip, thereby establishing a conduction path between the high voltage supply and the fluid being spun. In FIGS. 2, 5, 6A, and 6B of the '998 publication are shown various electrospinning arrangements in which an electrode is placed within a chamber containing the fluid to be spun, thereby establishing a conduction path between the high voltage supply and the fluid. The chamber communicates with a plurality of spinning tips. In any of those arrangements, significant current (typically greater than 1 μA per spinning tip) flows along with the spun polymer material. Conventional electrospinning fluids are deposited on metal target substrates so that current carried by the spun material can flow out of the substrate, thereby avoiding charge buildup on the target substrate. Electrospinning onto nonconductive or insulating substrates has proved problematic due to charge buildup on the insulating substrate that eventually suppresses the electrospinning process.